1. Field of the Invention
This invention relates to metallized ceramic articles and to a metallized conductor pattern directly and adherently bonded onto a surface of a ceramic substrate, and an improved process for producing the same. More particularly, this invention relates to a printed circuit pattern directly and adherently bonded onto a surface of a ceramic substrate, and an improved process for producing the same employing material in solution to promote adsorption of a catalyst for metal deposition.
2. Description of the Prior Art
Metallized conductor patterns on ceramic substrates have been widely used in the electronic industry. For many years, ceramics have been metallized by high cost processes such as fused metal-glass pastes and thin film vacuum deposition techniques. Attempts to reproducibly make circuit patterns by direct electroless deposition have not been successful due to poor adhesion of the metal films to the substrate and non-reproducible and non-uniform surface coverage.
Printed circuits on ceramics, including alumina, were described as early as 1947. See "Printed Circuit Techniques", National Bureau of Standards, Circular 468 (1947) and National Bureau of Standards, Misc. Pub. 192 (1948). One type, known as a thin film circuit, consists of a thin film of metal deposited on a ceramic substrate by one of the vacuum plating techniques. In these techniques, a chromium or molybdenum film, having a thickness of about 0.02 microns, acts as a bonding agent for copper or gold conductors. Photolithography is used to produce high resolution patterns etched from the thin metal film. Such conductive patterns may be electroplated up to 7 microns thick. Due to their high cost, thin film circuits have been limited to specialized applications such as high frequency applications and military applications where a high pattern resolution is vital.
Another type of printed circuit, known as a thick film circuit, consists of a metal and glass film fired on a ceramic substrate. Typically, the film has a thickness of about 15 microns. Thick film circuits have been widely used. Thick films are produced by screen printing in a circuit pattern with a paste containing a conductive metal powder and a glass frit in an organic carrier. After printing, the ceramic parts are fired in a furnace to burn off the carrier and sinter the conductive metal particles and fuse the glass, thereby forming glass-metal particle conductors. These conductors are firmly bonded to the ceramic by the glass and thus components may be attached to the conductors by soldering, wire bonding and the like.
Conductors in thick film circuits have only 30-60 percent of the conductivity of the pure metal. The high conductivity of pure metal is needed to provide interconnections for high speed logic circuits. Because conductors in thick film circuits do not have such high conductivity, they do not provide optimum interconnections for high speed logic circuits.
The minimum conductor width and the minimum space between conductors which can be obtained by screen printing and firing under special high quality procedures is 125 and 200 microns, respectively. However, under normal production conditions, these minima are 200 and 250 microns, respectively. For ceramic circuits requiring higher interconnection density, i.e., higher connectivity, multilayer techniques are used.
In the thick film multilayer process, a first layer of metal powder and glass frit is printed on a ceramic substrate and fired, typically at 850.degree. C, in a furnace. Then, an insulating dielectric layer is screened over the conductor pattern, leaving exposed only the points at which contact is made to the next layer of metallization. This dielectric pattern also is fired at 850.degree. C. Then, a second dielectric layer is printed and fired. Two layers of dielectric must be printed and fired to ensure that there are no pinholes. After the two layers of dielectric have been printed and fired, the next conductor layer is printed and fired making contact to the lower conductor layer as necessary through the openings left in the dielectric layers.
Typical multilayer ceramic packages contain two to six layers of metallization. Eight layers are not uncommon. For two layers of metallization, the substrate will be printed four times and fired at 850.degree. C. for four times. For three layers of metallization, the substrate will be repetitively printed and fired at 850.degree. C. seven times, and for four layer, thick film, multilayer ceramic, ten times. By the processes of the present invention, the same connectivity as a three or four layer film multilayer ceramic can be achieved by a two-sided, plated through hole, conductor pattern.
Attempts have been made to directly bond pure metal conductors to ceramic substrates including alumina in order to achieve higher conductivity for ceramic based circuit patterns (see U.S. Pat. No. 3,744,120, to Burgess et al. and U.S. Pat. No. 3,766,634 to Babcock et al.). Solid State Technology 18/5, 42 (1975) and U.S. Pat. No. 3,994,430, to Cusano et al., disclose a method for bonding copper sheets to alumina by heating the copper in air to form an oxide film on its surface. The copper sheet then is bonded through this film to alumina at a temperature between 1065.degree. C. and 1075.degree. C. in a nitrogen furnace. In order to obtain well adhered copper foil without blisters: (1) the copper foil must be carefully oxidized to a block surface; (2) the copper oxide thickness must be carefully controlled; (3) the amount of oxygen in the copper foil must be controlled; (4) the oxygen content of the nitrogen furnace must be maintained at a controlled level to maintain a very moderately oxidizing atmosphere; and (5) the temperature must be controlled within one percent. This extreme high temperature operation is difficult and expensive to tool, to operate and to control. If the aforementioned extremely stringent controls are not maintained, blisters and other adhesion failures in the copper foil to subblisters are apparent. In spite of the difficult operating conditions, the process of Cusano et al. is being introduced into commercial application because of the need for the metallized product.
Although the above described systems are commercially used, the need for direct, simple metallization of ceramics with a pure metal conductor, such as copper, has prompted a continuous series of patents and proposed procosses. See for example Apfelbach et al., Deutsches Patentschrift (DPS) No. 2,004,133,; Jostan, DPS No. 2,453,192 and DPS No. 2,453,277; and Steiner DPS No. 2,533,524. See also U.S. Pat. No. 3,296,012 to Statnecker which disclosed a method of producing a microporous surface for electrolessly plating alumina. Attempts to simply apply electroless metallization directly to ceramic substrates, have continually been tried and never been commercially successful. Even such toxic and corrosive materials as hydrogen fluoride were tried to allow the direct bonding of electroless metal to ceramics without extreme firing temperatures, Ameen et al., J. Electrochem. Soc., 120, 1518 (1973). However, the hydrofluoric etch gave poor strength due to excessive attack on the surface of the ceramic.
Another attempt, disclosed in U.S. Pat. No. 3,690,921 to Elmore, involved the use of molten sodium hydroxide to first etch a ceramic surface before sensitizing the surface with stannous chloride sensitizer, activating the surface in palladium chloride, and electrolessly plating the surface. Although the sodium hydroxide etch provided a metal film circuit with good bond strength, nonetheless, it did not achieve commercial production. The problem was poor surface coverage by the electrolessly deposited metal. Although the metal deposit usually covered 90% of the surface area or even better, this was insufficient. Any imperfection in a metal film can result in an open circuit, that is, a complete operating failure, if the imperfection occurs in a fine line conductor pattern.
U.S. Pat. No. 4,428,986 to Schachameyer discloses a method for direct autocatalytic plating of a metal film on beryllia. The method comprises uniformly roughening the surface of the beryllia by immersing the beryllia in a 50% sodium hydroxide solution at 250.degree. C. for 7 to 20 minutes, rinsing with water, etching the beryllia substrate with fluoroboric acid for 5 to 20 minutes, rinsing with water, immersing the beryllia in a solution of 5 g/l stannous chloride and 3N hydrochloric acid, rinsing with water, treating the beryllia with 0.1 g/l palladium chloride solution, rinsing with water, and then electrolessly plating nickel on the beryllia. However, the etching step removes the silica and magnesium from the grain boundaries of the beryllia, thereby weakening the beryllia surface. As a result, the method of Schachameyer was able to achieve only 250 psi (1.7 MPa) bond strength before the beryllia substrate broke. This bond strength is low, being approximately a third of the bond strength normal in thick film type circuits.
Other methods of forming printed circuit patterns on ceramic substrates are disclosed in U.S. Pat. Nos. 3,772,056, 3,772,078, 3,907,621, 3,925,578, 3,930,963, 3,959,547, 3,993,802 and 3,994,727. However, there is no teaching in these patents of how to solve the problem of poor surface coverage and inadequate bond strength to ceramic.
Ceramic packages have been widely used to protect microelectronic circuits such as integrated and hybrid circuits from contamination by the outside environment. A hermetic seal is required between the ceramic base of the package and its cover to exclude the outside environment. Heretofore hermetic seals have been provided by the following methods for ceramic dual-in-line packages utilized in the microelectronics industry. In one method, a refractory metal paste, usually moly-manganese, is applied to a ceramic base. The paste and the ceramic base are fired at 1500.degree.-1800.degree. C. in an inert or a reducing atmosphere in order to fuse the moly-manganese alloy to the ceramic surface. The refractory metal alloy then is plated with gold, or nickel and gold. A microelectronic circuit is inserted within a cavity of the ceramic base through an opening in the base. The microelectronic circuit is connected to feed throughs provided in the base. The opening in the ceramic base is hermetically sealed by covering it with a cover which is soldered in place around the periphery of the opening. Gold plated Kovar lids or ceramic lids provided with gold plated moly-manganese surfaces are used as covers. Soldering takes place in a nitrogen atmosphere using gold-tin solders or tin-silver-antimony solders. This process, while capable of producing good hermetic seals, is characterized by very high cost because of the difficulty of plating refractory metals and the very high firing temperatures.
Epoxy adhesives also have been used to provide seals between a ceramic package and a lid for the package. This method is unsatisfactory because organic vapors from the epoxy sealant may contaminate the microelectronic circuit.
Still another method of forming hermetic seals for a ceramic package is to use a low melting glass which is applied as a glass paste with a carrier solvent. The glass paste is applied to the sealing surface of the ceramic package and then is fused in a furnace. This method is unsatisfactory because it may seal solvents from the glass paste inside the package along with some of the glass itself. The high reliability of the circuit can be destroyed by contamination due to the solvent trapped in the glass interlayer.
An object of the present invention is to provide a method of applying a metal film to a ceramic substrate and obtaining excellent surface coverage and a bond strength of at least 3 MPa, preferably at least 5 MPa.
An object of the invention is to produce a metal plated ceramic substrate which may be used for fine line circuit applications with highly pure metal conductors.
An object of the invention is to provide an electrolessly deposited, direct bonded conductor having excellent adhesion to a ceramic substrate and a process for producing the metal coated ceramic substrate.
An object of the invention is to provide a plated ceramic substrate which has conductors suitable for interconnection for high speed logic circuits and a process for producing the plated substrate.
An object of the invention is to provide a two-sided plated ceramic substrate with a through hole conductor pattern and a conductor density comparable to a three or four layer thick film multilayer ceramic.
It is also an object of the invention to provide a method of joining ceramic articles or other metallized ceramics by soldering, brazing or welding.
An object of the invention is to provide a hermetic seal for ceramic articles.
Additional objects and advantages of the invention will be set forth in part in the description, or may be realized by practice of the invention, the objects and advantages being realized and attained by means of the methods, processes, instrumentalities and combinations particularly pointed out in the appended claims.